Biuret polyisocyanate is widely used in the production of coatings, adhesives, sealants, waterproofing agents, foams, elastomers, fiber processing agents and so on. The preparation method of the aliphatic polyisocyanates having biuret structures has been disclosed in the patent document DE 1101394B published in 1958. Other possible preparation methods are disclosed in the review paper “The synthesis of Aliphatic Polyisocyanates Containing Biuret, isocyanurate or uretdione backbones for use in coating”, J.prakt. Chem, 336, 1994, 185-200, and the advantages and drawbacks of these methods are discussed in the review paper.
The preparation methods of biuret polyisocyanate can be mainly divided into two categories: the first is a water method, in which urea is produced by the reaction of diisocyanate and excess water or water donors (for example tertiary monoalcohols, formic acid, crystalline hydrate etc.) and carbon dioxide is produced simultaneously, then biuret polyisocyanate is produced by the reaction of urea and excess diisocyanate; the second is a diisocyanate/diamine method, in which urea is produced directly by diisocyanate and insufficient amount of amine (for example primary amine and/or secondary amine), then biuret polyisocyanate is produced by the reaction of urea and excess diisocyanate. As stated in the above mentioned review paper (J.prakt. Chem, 336, 1994, 185-200), a variety of modifications of the above two methods have been developed and described.
Biuret polyisocyanates prepared by the water method have great monomer stability, i.e. they are hard to break into free diisocyanates, and good tolerance of dilution, i.e. the solution formed by said biuret polyisocyanates and diluents are not likely to be cloudy and generate sediments; and the condition of the preparation process of the water method is relatively mild, and the color number of the obtained products is relative low, thus the method is widely used in manufacturing.
However, in the preparation of the water method, solid state polyurea is very likely to be produced during the reaction process because of the poor compatibility and the insufficient contact of liquid water and diisocyanates; furthermore, the tail gas produced during the reaction still contains a certain amount of diisocyanates and water vapour, and the water vapour can be reacted with a part of diisocyanate gas, solid state polyurea is also easily produced by such reaction, resulting in the blocking of condensers and exhaust pipes and a loss of a part of diisocyanate materials; when solvents are used in the preparation method, polyurea in the condensate of the tail gas will be condensed and refluxed to the products with the solvents. Thus the products will contain polyurea, which will result in poor homogeneity, high cloudiness and white appearance of the products, thus influencing the property of the products, for example the anti-corrosion of the coatings prepared by such products and so on.
In the US patent document U.S. Pat. No. 4,028,392A, it is disclosed that in the presence of hydrophilic organic solvents such as trialkyl phosphate and ethylene glycol methyl ether acetate, diisocyanate is reacted with water to produce biuret polyisocyanates. In European patent document EP0259233A, it is disclosed that in the presence of at least one carboxylic acid and/or carboxylic acid anhydride as catalysts, diisocyanate is reacted with water; in said method, it is also disclosed that methyl phosphate and/or ethyl phosphate and alkoxyl alkyl carboxylate can be used simultaneously as solubilizers, to increase the solubility of water in the solution of diisocyanate and the catalyst. In the above two preparation methods, due to the use of a necessary amount of solvents or solvent mixtures, a relative low biuret polyisocyanate space-time yield is obtained compared with the condition that no solvents are used, and the polyurea produced in a tail gas is condensed and refluxed to the products with solvents, resulting in a cloudy product with white appearance obtained after separation. Moreover, devices and energy-consumption are increased and a more complex operation is required due to the use of solvents, thus more complex distillation operation is required to separate solvents.
In German patent document DE2918739A1, it is disclosed a method for preparing polyisocyanate having biuret structure by the reaction of hexamethylene diisocyanate (HDI) and water, wherein water is mixed with air and/or inert gas in the form of vapour and then is added to the mixed solution of HDI and a catalyst with the temperature of 110-130° C., the reaction is carried out under 150-170° C. The drawback of said method is, as a single reactor is used for the operation of mixing of gas and liquid, the conversion rate of water vapour is not high enough, the tail gas generated during the reaction process produce a large amount of solid polyurea, resulting in the blocking of the device parts, especially the tail gas pipes; meanwhile, the tail gas is not treated, resulting in a great loss of diisocyanate materials.
In the Chinese patent document CN101475680A, it is disclosed a method of synthesizing hexamethylene diisocyanate biuret by spraying, wherein biuret polyiscyanate is prepared by the reaction of hexamethylene diisocyanate and water that is in the form of fogdrop achieved by the use of high pressure. As water cannot be dispersed timely and sufficiently when liquid drop-like water is reacted with hexamethylene diisocyanate, polyurea is formed unavoidably, thus resulting in highly cloudy products with white appearance obtained after separation.
In the Chinese patent document CN101072805A, it is disclosed a method for preparing a storage-stable colorless polyisocyanates having biuret groups. A single reactor is used in said method, and water participated in the reaction is in the form of water vapour to solve the problem of water dispersing. Meanwhile, cold hexamethylene diisocyanate is used to wash tail gas, thus decreasing the loss of the material, hexamethylene diisocyanate. However, as a single reactor is used in operation in said method, the conversion rate of water is not high enough, resulting in a large amount of water left in the returning condensate, these water are reacted with diisocyanate to produce polyurea, leading to products with high cloudiness obtained after separation.
Because the current biuret polyisocyanate preparation methods have many drawbacks, a new biuret polyisocyanate preparation method that will not cause the blocking of the exhaust pipes of condensers during the reaction process and has a low diisocyanate material loss and will produce products with low cloudiness and transparent appearance is required.